1. Field of the Invention
The present invention generally relates to a holographic recording apparatus for recording interference fringes on a hologram recording material.
2. Description of the Prior Art
Recently, with the spread of laser beam printers, bar-code readers and the like, there has been a considerable demand for devices utilizing optical technology. Holographic optical elements are very attractive because they are suitable for mass production with low cost, and are capable of easily producing special aberration waves, as compared with optical lenses. A holographic optical element is a diffraction grating having a periodic structure formed on a hologram recording material. It is possible to make holographic optical elements have various optical characteristics by adjusting, for example, spatial frequencies, that is, by adjusting intervals between fringes.
Two methods are known which produce holographic optical elements having fringe patterns on a hologram recording material. A first conventional production method utilizes interference of two or more light beams. The first method is called a holographic exposure method. A second conventional production method uses an electron or laser beam exposure apparatus in which interference fringes are directly formed on a hologram recording material in accordance with phases calculated by a computer beforehand. The above second method is called a computer generated hologram method.
The holographic exposure method is capable of producing a large-area hologram having a high spatial frequency in a short time. However, it is very difficult to produce a holographic optical element having desired interference fringes. The computer generated hologram method is capable of forming desired fringe patterns, but is not capable of producing interference patterns having high spatial frequencies. For example, the holographic exposure method is capable of producing an interference pattern having a spatial frequency of 1000-2000 /mm, while the computer generated hologram method has an ability to produce an interference pattern having a spatial frequency between 100 and a few hundred per millimeter. In order to obtain an interference pattern having a high spatial frequency, it is necessary to scale down an interference pattern formed by the computer generated hologram method. Further, the computer generated hologram method cannot form a large hologram area. A maximum hologram area is equal to a few square millimeters. Furthermore, it takes a long time to draw interference patterns by the computer generated hologram method.